1. Field of the Invention
The present invention relates to techniques for measuring optical systems offering both high dynamic range and verifiable high-accuracy.
2. State of the Art
Optical metrology is the study of optical measurements. An area of optical metrology relevant to the present invention is the use of an interferometer to measure the quality of an optical system, such as a mirror or a lens.
One important recent application of optical metrology is the testing of projection optics for photolithography systems. Modern photolithography systems used to fabricate integrated circuits must continually image smaller features. To do so, systems are confronted with the diffraction limit of the light employed to image a pattern provided in a reticle. To meet this challenge, photolithographic systems must employ successively shorter wavelengths. Over the history of integrated circuit fabrication technology, photolithography systems have moved from visible to ultraviolet and may eventually move to x-ray radiation.
Because of the increasing difficulties posed by directly imaging a reticle pattern onto a wafer, it is desirable to use projection optics in lithography systems. Such systems include lenses or other optical elements that reduce the reticle images and project them onto the wafer surface. This allows reticles to retain larger feature sizes, thus reducing the expense of generating the reticle itself.
As with all optical imaging systems, various aberrations such as spherical aberration, astigmatism, and coma may be present. These aberrations must be identified during the fabrication and/or testing of the projection optics, or the projection optics would introduce substantial blurring in the image projected onto the wafer.
In order to test the projection optics for various aberrations, interferometers may be employed. Conventional interferometers, based upon the Michelson design for example, employ a single coherent light source (at an object plane) which is split into a test wave and a reference wave. The test wave passes through the optic under test and the reference wave avoids that optic. The test and reference waves are recombined to generate an interference pattern or interferogram. Analysis of the interferogram and the resultant wavefront can reveal the presence of aberrations.
The reference wave of the interferometer should be xe2x80x9cperfectxe2x80x9d; that is, it should be simple and well characterized, such as a plane or spherical wave. Unfortunately, beam splitters and other optics through which the reference beam passes introduce some deviations from perfection. Thus, the interferogram never solely represents the condition of the test optic. It always contains some artifacts from the optical system through which the reference wave passes. While these artifacts, in theory, can be separated from the measured aberrations, it is usually impossible to know that a subtraction produces a truly clean result.
To address this problem, xe2x80x9cpoint diffraction interferometersxe2x80x9d have been developed. An example of a point diffraction interferometer is the phase-shifting point diffraction interferometer (PS/PDI) described in the article H. Medecki et al., xe2x80x9cPhase-Shifting Point Diffraction Interferometerxe2x80x9d, Optics Letters, 21(19), 1526-28 (1996), and in the U.S. Pat. No. 5,835,217 xe2x80x9cPhase-Shifting Point Diffraction Interferometerxe2x80x9d, Inventor Hector Medecki, which are both incorporated herein by reference. Referring to FIG. 3, in this prior art phase-shifting point diffraction interferometer 2 a focused illumination source 4 is spatially filtered using a pinhole 6 placed in the object plane of the optical system 8 (hereafter called optic) under test. The pinhole diffracts a spherical illuminating wavefront that is focused in the image-plane of the test optic. A coarse grating beamsplitter 10 (e.g. a transmission grating) placed either before or following the test optic divides the beam into multiple overlapping diffraction orders. The series of diffracted orders each containing the aberrations of the optical system, are focused in the vicinity of the image-plane with a small lateral separation. The separation is determined by the wavelength, and the grating""s pitch and position. A patterned opaque and transparent mask, called the PS/PDI mask 12, is placed in the image-plane where it allows only two of the orders to be transmitted. One of those beams, called the test beam, is selected to pass through a relatively large window and propagate on to reach a detector 14 placed significantly beyond the image plane. A second beam, called the reference beam, is spatially filtered by a pinhole, called the reference pinhole, in the PS/PDI mask. Via pinhole diffraction, the spatially filtered reference beam, becomes a spherical reference. The reference beam combines with the unfiltered test beam creating a pattern of light and dark fringes (an interferogram) that reveals the aberrations in the test optic. Although the PS/PDI has been proven to have high-accuracy, broad applicability of the device is severely limited by its small dynamic range.
In contrast to the PS/PDI, lateral shearing interferometry (LSI) is a well-known and reliable method of optics testing that is characterized by a larger dynamic range. The LSI 20 shown in FIG. 4 requires one less optical component than the PS/PDI but operates in nearly the same geometry. An incident focused beam 24 is spatially filtered in the object plane by a pinhole in object mask 26, thus illuminating the test optic 28 with a spherical wave. A grating beamsplitter 30 is placed near the image-plane, where the illuminating beam is focused. The diffracted orders propagate unfiltered to the detector where they overlap. While the central zeroth-order propagates to the detector in the same manner as it would if the grating were not present, the diffracted orders propagate with an angular shear, leading to a (typically) small lateral displacement at the detector 34. In this way, the test beam is compared with multiple sheared copies of itself. Analysis of the resultant interference pattern reveals an approximation to the gradient of the test wavefront, or the derivative in the direction of the shear. The original wavefront can be recovered by combining gradients from two (or more) directions. While the LSI has simplified geometry and operation relative to the PS/PDI, analysis of the PS/PDI data is more straightforward.
One primary difficulty encountered in the operation of the PS/PDI is alignment. For EUV optical systems with resolution on the order of 100 nm, the alignment of the focused beam onto the similarly sized reference pinhole poses a significant challenge. The alignment can be judged based on the visibility or contrast of the observed interference pattern: peak contrast indicates optimal alignment and will yield maximum signal-to-noise ratio in the measurement. With the test beam passing through a relatively large window (compared to the size of the focused beam), the fine alignment mainly affects the reference beam; its intensity rises and falls as its position on the reference pinhole changes.
For a similar reason, the quality (aberration magnitude) of the test optic also affects the achievable fringe contrast. When aberrations limit an optics ability to form a small focused beam, the light intensity transmitted through the reference pinhole suffers. With only about one-wave of rms aberration magnitude (wavefront error), the focused intensity can drop low enough to make interferogram analysis very difficult if not impossible. Furthermore, the alignment becomes significantly more difficult to perform. In this way, we say the dynamic range of the PS/PDI is limited. Its useful measurement range includes only optical systems in nearly perfect alignment.
In contrast to the PS/PDI, operation of the LSI has more relaxed alignment and dynamic range requirements. A large transmission grating is placed near the image plane of the test optic. In typical operation the grating is placed far enough from focus that many (10 to 100) grating lines are illuminated. In this configuration, there is no critical lateral alignment. Nor does the measurement suffer if the test optic has more than one wave of rms aberration magnitude.
In the LSI, the lateral alignment is not critical because the grating can be very large and because it is a periodic structure. Therefore, moving it laterally by one period has no effect on the recorded image. However, there is a restriction on the longitudinal position: this restriction is dictated by what are known as the Talbot planes of the grating. The source must be placed close to or in a Talbot plane of the grating. These planes are evenly spaced and are separated by a distance d2/xcex where d is the grating pitch (or period) and xcex is the wavelength of the light. The dynamic range of the LSI is significantly larger than that of the PS/PDI. The factors that most significantly limit the dynamic range of the shearing interferometer are a restriction on the wavefront curvature, which is similar to the Talbot condition, and a restriction on the maximum fringe density (and with it a restriction on the maximum wavefront departure from sphericity) that the CCD detector can resolve. (If there are too many fringes, or the fringe density is too high, the limited number of pixels available will not be able to resolve them.)
Data analysis in lateral shearing interferometry requires that the original wavefront be reconstructed from more than one sheared wavefront measurement. Because the shearing measurement, using a conventional one-dimensional grating, yields the slope of the wavefront in only one direction, more than one slope measurement (recorded with more than one grating orientation) is required to unambiguously reconstruct the original wavefront. Using a two-dimensional grating pattern enables two directions of wavefront shear to be recorded at once. Use of a two-dimensional grating also reduces measurement uncertainties caused by unknown longitudinal position changes between measurements performed with different gratings. For these reasons the two-dimensional grating is advantageous for LSI measurements. Phase-shifting is provided by moving the grating at an angle relative to the rulings of the grating. Phase-shifting improves the efficiency and accuracy of the system.
The difficult and unavoidable task of optical system alignment involves reducing the aberrations in an assembled optical system by adjusting the free mechanical parameters. Feedback is usually provided by some means of wavefront-measuring interferometry. In such circumstances, an essential requirement of the interferometry method or tool is that it have both high dynamic range (the ability to measure the system during early or initial phase of alignment when the aberrations are large) and high accuracy, which ensures meaningful results. At present, there is no single technique that offers both high dynamic range and verifiable high-accuracy for the measurement of EUV optical systems.
The present invention is based in part on the recognition that lateral shearing interferometry can be incorporated in conjunction with the PD/PDI to achieve both large dynamic range and high-accuracy. The accuracy of the PS/PDI can be measured in situ, and systematic errors can be identified. This improves the accuracy of concomitant shearing measurements. This combination of the strengths of two methods is well-suited, but not limited, to the measurement of optical systems designed for the extreme ultraviolet (EUV) spectral regions (near 13-nm wavelength), where high-accuracy measurement tools with large dynamic range do not yet exist.
In one embodiment, the invention is directed to a hybrid shearing and point diffraction interferometer system for testing an optical element that is positioned along an optical path which includes:
a source of electromagnetic energy in the optical path;
a first beam splitter that is secured to a device that includes means for maneuvering the first beam splitter in a first position wherein the first beam splitter is in the optical path for dividing light from the source into a reference beam and a test beam and in a second position wherein the first beam splitter is outside the optical path;
a hybrid mask which includes a first section that defines a test window and at least one reference pinhole and a second section that defines a second beam splitter wherein the hybrid mask is secured to a device that includes means for maneuvering either the first section or the second section into the optical path positioned in an image plane that is created by the optical element; and
a detector positioned after the hybrid mask along the optical path. The system is preferably configured so that the first section of the hybrid mask is positioned in the optical path when the first beam splitter is positioned in the optical path.
In operation, before or after an optical element to be tested is positioned in place in the hybrid device, the hybrid device is configured to perform lateral shearing interferometry by maneuvering the first beam splitter out of the optical path and maneuvering the first section of the hybrid mask into the optical path. Following lateral shearing interferometry, the hybrid device is reconfigured by maneuvering the first beam splitter into the optical path and maneuvering the second section of the hybrid mask comprising the second beam splitter into the optical path. Thereafter, phase-shifting point diffraction interferometry is performed.